Field of the Invention
The present invention relates to an image processing apparatus, an image capturing apparatus, and a control method, and in particular to a technique for detecting a decrease in pixel signal output due to an external factor.
Description of the Related Art
Some image capturing apparatuses such as digital cameras are capable of storing, as light field information, an intensity distribution of a light flux that is reflected from a subject and incident on an image sensing plane together with information on an incident direction. Such an image capturing apparatus can use the light field information stored by shooting to reproduce an intensity distribution of the light flux at a predetermined focal position after the shooting, and to generate (reconstruct) an image (reconstruction image) that is in focus at the corresponding subject distance (Japanese Patent Laid-Open No. 2007-4471).
However, an output of a pixel signal obtained by shooting using an image capturing apparatus may differ from a signal output that is supposed to be obtained, corresponding to a light flux that is to be used for image formation on pixels. Furthermore, in the image capturing apparatus disclosed in Japanese Patent Laid-Open No. 2007-4471, if such a decrease in signal output occurs, a correct intensity distribution cannot be reproduced when the reconstruction image is generated using the acquired light field information, resulting in a deterioration of an image quality of the reconstruction image.
One factor of varying a pixel signal output is a local sensitivity defect occurring in a semiconductor (photoelectric conversion element) that constitutes an image sensor. Such a sensitivity defect includes an initial defect that occurs in a manufacturing process of the image sensor, and a latter defect such as aging. A pixel that causes such a sensitivity defect can be detected by, for example, a method of detecting occurrence of a dark current in a light shielding state.
Furthermore, a signal output may also vary due to the existence of a foreign substance incorporated in the image capturing apparatus on a light path of a reflected light flux. For example, when a microlens is arranged on the light path as in Japanese Patent Laid-Open No. 2007-4471, a signal output may decrease due to a foreign substance such as dust or mote that is attached to the microlens. Specifically, if the foreign substance is attached to the microlens, a light flux that passes through the microlens will partially be shielded, thus causing a decrease in signal output. Because the light field information is particularly acquired by the microlens separating an incident light flux into different multiple incident directions, a decrease in signal output occurs in at least some of pixels of a pixel group associated with the microlens. A decrease in signal output may also occur, when the microlens is used, due to a scratch, crack, or the like on the microlens as well.
However, it is difficult to determine whether the factor of varying a pixel signal output is a sensitivity defect of the semiconductor or an external factor such as a scratch of the microlens or an attached foreign substance.